1. Field of the Invention
The present invention relates to a fixing apparatus.
2. Description of the Related Art
Heretofore, heating apparatuses that drive a heater with phase control using an AC power supply as the drive source are known. With conventional phase control, there is a problem in that harmonic noise levels increase when power varies widely. For example, if power is supplied at a duty ratio of 50%, the power supply is switched on/off at timings at which the sine wave peaks at phase angles of 90 and 270 degrees, as is shown in 11a in FIG. 11. In this case, noise is produced because of supplied power suddenly changing to zero.
In response, Japanese Patent Laid-Open No. 2006-73431 discloses a technique according to which on/off control of the heater is not performed at timings near the peaks of the sine wave indicating input power, in order to reduce sudden changes in power variation when performing phase control.
However, the following problem exists with the technique of Japanese Patent Laid-Open No. 2006-73431. When printing is performed continuously for an extended period of time, power input to the heater stabilizes to a substantially constant value due to the fixing apparatus warming up sufficiently. At this time, the harmonic levels of a specific order increase despite control being performed so as to avoid using heater current application start timings at which harmonic levels will be adversely affected, due to the same current application start timings being repeatedly used.